Obama's has jumped out to an average lead of 3.1 percent points in 10 national polls taken since Sept. 4. That's triple the 1.1 percentage-point edge Obama held in polling conducted between Romney's selection of Wisconsin Congressman Paul Ryan as his running mate and the end of the Republican National Convention.

"Unless there is a crisis-driven shift in opinion, the race will probably stay close, with a slight advantage to Obama," said John J. Pitney, Jr., a professor of American politics at Claremont McKenna College.

But the president's narrow popular vote edge is magnified in the Electoral College. Obama holds a lead in the polls in 11 of the 12 battleground states being contested by both candidates.

Swing state advantage

An analysis by Real-ClearPolitics.com, a nonpartisan web site, shows the Democratic incumbent currently leading in states (and the District of Columbia) with 332 electoral votes - 62 more than is needed to ensure his re-election - while Romney is ahead in states with 206 electoral votes. Romney is leading in only two states won by Obama in 2008: Indiana and North Carolina.

The Republican nominee needs to capture at least eight of 12 swing states captured four years ago by Obama to have a chance in an Electoral College that favors Democrats because of the president's dominance in megastates such as California, New York, Illinois and Pennsylvania. Texas is the only one of the nation's five most populous states that is safely in the Romney column.

Romney's campaign argues that Obama is benefiting from a temporary post-convention blip that will disappear in coming weeks. But the GOP nominee has been on the defensive this week over comments he made in the aftermath of attacks on U.S. diplomatic compounds in Libya and Egypt. And his campaign was forced to clarify the candidate's view of the middle class Friday after Romney told ABC News that "middle income is $200,000 to $250,000 and less."

Debates important

Presidential scholars say Romney is unlikely to win the election unless he makes a breakthrough in the upcoming debates or Obama suffers an unexpected political reversal.

"The first debate (in Denver on Oct. 3) could tell the tale for this campaign," said Cindy Rugeley, a Texas Tech University political science professor. "Gov. Romney must do something terribly right or President Obama must do something terribly wrong."

Most distressing for Republicans is Obama's continuing lead in Ohio, a bellwether state carried by every winning Republican presidential candidate in U.S. history. Romney has led in only six of the 36 Ohio presidential polls this year, and the Democratic nominee is running ahead in all four polls taken since the GOP convention ended, by an average margin of 4.25 percentage points.

"It is all-important that Obama is leading by a little, and sometimes more, in most, if not all, of the states both campaigns see as must-win," said Cal Jillson, a political scientist at Southern Methodist University. "Things can change between now and election day, but if Obama wins Florida and Ohio - and he is leading narrowly in both now - it is hard to see a path to the White House for Romney."